Any assassin's creed from the last 10 years, probs gonna get hate for that but they are just so average to me.

A hell of a lot of Ubisoft open-world slop released around and in the 2010s.

Kingdoms of Amalur: Reckoning

Had all the individual makings of an exceptional game (with input from Todd Macfarlane, R A Salvatore and Grant Kirkhope), and while it was definitely enjoyable enough - it lacked any wow-factor whatsoever, winding up an otherwise forgettable 7/10.

This is probably more subjective than best/worst. So...

Vanilla Skyrim.

Every Halloween, I play this Xbox 360 (I think it's also on PC now) game called Bullet Witch.

Basically a third-person shooter with postapocalyptic supernatural horror theme. You play as a witch who shoots zombies and weird creatures with a magic machine gun broom thing. Also you get spells. Some are bloody awesome.

This game is peak Xbox 360 to the core. The distinct memorable thing about it is that I can actually list good and bad things about it. Level design varies between meh and decent. Some of the particular setpieces are pretty awesome though. (You get to fight at an airport, and you get to do a boss fight at the top of the plane mid-flight!) Spells are fun. The mega-spells are hella fun. (Just call up lightning and watch stuff explode.) Shooting is kinda jank but it works. Jank is explained by lore. (Why is friendly fire not a thing? Well, you see, this is a magic machine gun broom thing, so bullets dodge the civilians and allies by ~*~magic~*~.) Enemy designs are nothing to write home about at first glance, but are actually kinda memorable. (You first meet up the zombies and hey, they're talking zombies. With military helmets and guns. Like, what? You don't see this every day.) There are some things that seem just not very well designed, like there's these gigantic enemies that serve as minibosses and they're a lot less scary when you note the AI is probably bugged and they often just decide to stand at place for a while and eat a lot of bullets.

I got this thing in the bargain bin. It's a zombie shooty game that's perfect for Halloween so that's what I use it for. That's all it does. That's all I could ask it for. And it's fine at it.

The Technomancer

Anything ubisoft makes. Or generally most things big companies make to cater biggest possible amount of people.

Ghost Wire: Tokyo.

It sells itself on cool aesthetics, but the moment you get past that you realise it's just a very, very generic open world shooter with incredibly bland and boring shooting layered over an impressively faithful recreation of Shinjuku. And even the aesthetics wear thin very quickly, being largely just a whole lot of "Hey I know that anime" level stuff cribbed from Japanese culture. The game is mostly just running around a map collecting stuff.

The incredible adventures of Van Helsing. Decent rpg with loot but was fairly imbalanced among character classes. The real winner was the mini tower defense games in it and the spinoff tower defense game, imo.

Square Enix games (FInal Fantasy, Neir Automata, Sleeping Dogs. Tomb Raiders)

They are all... good - certainly not bad games But nothing makes them... great

This is a tough question because it's like asking "What's the most forgettable game you've ever played?" I can remember some of the best and worst games I've ever played, but mediocre games are explicitly not interesting.

That said, the first one that came to mind for me was Starshot: Space Circus Fever for N64. It's just a very generic late-'90s collectathon platformer. It's hard to be mad at it, because it's not terrible or anything, there's just no reason to play it. If you've got an N64, there's Mario, Banjo, Rayman, even B- and C-tier stuff like Gex and Chameleon Twist. There's hidden gems like Space Station Silicon Valley or Rocket: Robot on Wheels.

That last one is the only reason I played Starshot, I saw it clearanced at a used game store and was like "Oh yeah, I remember hearing this game was good," but it turned out I was thinking of Rocket. That game actually is good, while Starshot is just fine.

Dual nominations for Paper Mario: Sticker Star & Paper Mario: Color Splash. The only thing I really remember about them is that I played them and they left me without any feelings about them whatsoever.

From a gameplay perspective GTA has been mid for ages.

Team Fortress 2:

I'd say its gameplay is more "robust" than special. Like you can have any and every kind of fight in TF2 but none of it is more special than an FPS that specializes in any game mode.

The Halo series.

I like shooters, so I got the full bundle and I tried hard to like it.

None of the games gave me a lasting impression. The plot didn't stick with me, the enemies were weird, the guns felt weak and flimsy, the rooms kept repeating in some sections and it got very boring. There were some fun bits with the vehicles, etc., but overall the experience was... pretty much average.

I was expecting something like the Half-Life series, but this wasn't it.

Elex 1 and Elex 2

I think Halo Infinite qualifies, I played the multiplayer waaay back when it released so things may have drastically changed (haven't heard of it being the case);
it didn't / doesn't do anything that no other game does, nor did / does it do anything particularly well nor better than its competitors (including every Halo from Bungie).

I did watch a walkthrough of the campaign, and it doesn't look particularly engaging either.

Warhammer 40,000: Space Marine.